Offshore wind farms: Low loss solutions for transferring current

To exactly determine the unavoidable losses also in this case, and to set up a metrological infrastructure for a future network of direct current transfer paths, a European cooperation project has been launched in which a great number of metrology institutes is involved. The starting signal for this project came from Braunschweig, from a close cooperation between the Technical University (TU) and the Physikalisch-Technische Bundesanstalt (PTB) which are now both intensively involved in the new project.

Already now the networks are getting narrow. The integrated European high-voltage network is used to complete capacity. Connecting other energy generators -- such as wind power stations, hydropower plants or solar power plants -- is hardly possible any more; new ways for the current must be found. As the construction of high-voltage power lines is often thwarted by protests from residents and does not come into consideration for offshore plants anyway, subterranean cables must be used. What is being planned is nothing less than a new transfer network all over Europe. If current is to be transferred via cables -- and over such large distances -- this is possible only with direct current, because in that case, the losses are lower.

The networks used so far, however, work with alternating current. This has been the case since the end of the 19th century, when the decision was taken to use alternating current instead of direct current for the large-area distribution of current. The decision was based on the fact that -- at that time -- a simple and effective current transfer with high voltage and, thus, without great losses, was possible only for alternating current. But this was long ago. And since in semiconductor electronics, new heavy-duty circuit breakers have been developed which allow even high powers to be converted efficiently from direct current to alternating current and vice versa, there is, in principle, nothing standing in the way of the new direct current network.

Regenerative energy generation plants supplying their current via direct current cables are already in existence. When the current is fed into the integrated high-voltage network, it must be transformed into alternating current. "Up to now it has only been possible to measure the amount of the transport losses for alternating current -- the whole range of direct current is still a 'Black Box' for us," declares Wolfgang Lucas, who is responsible for the project at PTB.

This is due to the fact that standardized measuring techniques have existed so far only for alternating current. It is high time to extend the whole metrological infrastructure also to direct current technology. This infrastructure has a scientific side such as, for example, the development of increasingly better measuring instruments. And there is a bureaucratic side -- namely an efficient system for checking these measuring instruments. In Germany, PTB is the highest authority for type examinations, for example for electricity meters (up to now, however, only for alternating current). "But we are already well prepared for direct current technology," points out Lucas.

In the project, Braunschweig Technical University has been assigned the task of investigating, in closer detail, the losses of the heavy-duty circuit breakers in the converters of the high-voltage direct current stations, and to reduce them. The head of the project, Michael Kurrat, emphasizes: "The starting signal came from Braunschweig, from the extraordinarily efficient cooperation between TU and PTB in exactly this field."

The other participants in the project are the national metrology institutes of Sweden (project management), the Netherlands, Turkey, Italy, Great Britain, and Finland. The project is financed within the scope of the European Metrology Research Programme (EMRP), with which the European metrology institutes coordinate their research. The project has been officially launched on September 1st and will, for the time being, be operated for three years. The participants expect decisive impulses for the European high-voltage direct current network -- as they are required for projects like "Desertec." Other areas in which direct current plays a role (e.g.: photovoltaic facilities or electric cars) will also benefit from the project.